This chapter considers the nature of constitutionalism. It begins by examining accounts of constitutionalism that present the doctrine as a constraint on state power. These understandings of constitutionalism, negative constitutionalism, rest on accounts of the state that present that institution as a threat to its people, and constitutions as sets of rules that are imposed on, and constrain, the state, mitigating this danger. This understanding of constitutionalism misses an important aspect of the doctrine and rests on a misleading account of states and constitutions. Constitutionalism requires the creation of an effective and competent set of state institutions; it has a positive dimension. In contrast to negative constitutionalism, positive constitutionalism recognizes that the state exists to benefit its people, and the constitution is that set of rules that empowers and constructs state institutions. The account of constitutionalism provided in this chapter sets the agenda for the rest of the book. It locates constitutionalism within constitutional theory, and examines the connection between constitutionalism and the principles discussed in the following chapters.